Topology-Controlled Excited-State Relaxation in Conjugated Cycloparaphenylenes.

Abstract

The topology of cyclic conjugated molecules plays a fundamental role in determining how electronic excitation evolves following photoexcitation. Connecting cyclic π-conjugated systems to linear conjugated extensions provides an effective strategy for tuning their optoelectronic and excited-state dynamical properties through molecular connectivity. Here, we employ nonadiabatic excited-state molecular dynamics simulations to investigate the relaxation mechanisms of cycloparaphenylenes connected through linear conjugated extensions, creating linear and disjoint architectures through ortho/meta-substitution patterns. These two topologies exhibit very similar absorption spectra and ultrafast relaxation to the lowest excited state, while displaying markedly different patterns of excited-state reorganization. In both systems, the initially delocalized excitation evolves toward a localized excited state. However, analysis of the time-dependent transition density reveals that the disjoint architecture preferentially channels excitation toward the ortho-associated region of the nanohoop, whereas the linear analogue maintains a more balanced participation of ortho- and meta-associated regions. Flux analysis further shows that the buildup of transition density in the final localized state is predominantly fed through ortho-related pathways in the disjoint topology, while both pathways contribute more evenly in the linear architecture. These results show that the different connectivity patterns can bias the spatial redistribution of electronic excitation without substantially altering global relaxation dynamics. For the two architectures studied here, the different ortho/meta-connectivity patterns therefore influence the preferred spatial localization of the excited state, providing a mechanistic basis for tuning photoinduced excitation redistribution through molecular connectivity.